A micro electro mechanical system (MEMS) device is made up of small parts created by fine processing technology for semiconductors. An optical switch that switches optical paths in an optical communication system uses a MEMS device (see, for example, Japanese Laid-Open Patent Application Publication No. 2004-70053).
The MEMS device applies high voltage to electrodes to gather electrical charge at each electrode and yields a mechanical displacement by electrostatic force. Since the extent of the mechanical displacement can be adjusted by the voltage applied, the MEMS device is used for many purposes in addition to optical switches.
FIG. 15 is a perspective view of electrodes of a conventional MEMS device. As depicted in FIG. 15, the MEMS device includes a fixed electrode 1510 and a movable electrode 1520. On the fixed electrode 1510 and the movable electrode 1520, electrodes 1511-1514 and 1521-1524 are arranged in a comb-like configuration.
When voltage is applied to the fixed electrode 1510 and the movable electrode 1520, the electrodes 1511-1514 take positive charge and the electrodes 1521-1524 take negative charge. In this way, electrostatic force works between the electrodes 1511-1514 and the electrodes 1521-1524, whereby the movable electrode 1520 moves in the direction indicated by an arrow 1530 with respect to the fixed electrode 1510.
When the MEMS device is applied to an optical switch, a mirror is fixed to the movable electrode 1520. Voltage is applied to the fixed electrode 1510 and the movable electrode 1520 to displace the movable electrode 1520 and change the angle at which the mirror reflects light. As a result, the optical path is switched by controlling the voltage for the fixed electrode 1510 and the movable electrode 1520.
However, if the voltage applied is increased in order to increase the amount of displacement of the movable electrode 1520 and improve performance of the MEMS device, occasionally spark discharge occurs between the fixed electrode 1510 and the movable electrode 1520. The spark discharge results in unexpected movement of the movable electrode 1520 or damage to the fixed electrode 1510 and the movable electrode 1520.
FIG. 16 is an explanatory diagram for the principle of spark discharge. As depicted in FIG. 16, increasing the voltage applied between electrodes 1601 and 1602 accelerates an electron 1612 and a gas molecule 1611 colliding with the electron 1612 is ionized. As reference numeral 1620 indicates, a positively charged ion 1621 generated by the ionization collides with the electrode 1602, a negative electrode. A large, instantaneous electric current due to secondary electron emission caused by the collision is called spark discharge.
FIG. 17 is an explanatory diagram illustrating spark discharge occurring in the conventional MEMS device. As indicated by reference numeral 1701, when spark discharge occurs between an electrode 1512 and an electrode 1521, charges on electrodes near the electrodes 1512 and 1521 (or on all electrodes) disappear by recombination.
Thus, electrostatic attractive force displacing the movable electrode 1520 drops instantaneously. An instantaneous drop in electrostatic attractive force causes unexpected behavior of the movable electrode 1520. In particular, when the MEMS device is used as an optical switch in an optical network, the spark discharge causes unfavorable displacement of the movable electrode 1520 and disturbs the optical path, resulting in a communications breakdown on a network.
In addition, once the spark discharge occurs between the electrodes 1512 and 1521, it takes time to supply the electrodes 1512 and 1521 with charges from other electrodes, a power source, or ground, and to reduce a potential difference between the electrodes 1512 and 1521. As a result, the spark discharge occurs for a long time and consequently, it takes time for the movable electrode 1520 to return to an original position. Furthermore, since the spark discharge occurs for a long time, current due to the discharge increases, whereby the displacement of the movable electrode 1520 becomes larger.
FIG. 18 is a circuit diagram that is equivalent to the MEMS device depicted in FIG. 17. The fixed electrode 1510 and the movable electrode 1520 of the MEMS device are equivalent to multiple condensers connected in parallel. As depicted in FIG. 18, the condensers connected in parallel can be thought of as one large condenser 1801.
The spark discharge between the fixed electrode 1510 and the movable electrode 1520 is equivalent to a state where a switch 1802, which is connected to both ends of the condenser 1801, is instantaneously turned on. As the switch 1802 is turned on, charges on the condenser run through the switch 1802 and disappear.
FIG. 19 is a graph depicting the displacement of the mirror of the conventional MEMS device due to spark discharge. In FIG. 19, the horizontal axis is time [sec]. The vertical axis is an angle [°] of rotation of a mirror fixed on the movable electrode 1520. Reference numeral 1901 indicates a point at which the spark discharge occurred. As depicted in FIG. 19, the maximum displacement of the mirror fixed on the movable electrode 1520 occurs when the spark discharge occurs, and the displacement decreases over time.
One way to deal with this problem is to cover the entire surface of the fixed electrode 1510 and the movable electrode 1520 with insulating film, preventing secondary electrons from being generated at the collision of positive ions against electrodes. One example is a vacuum deposition method where coating material is heated to a high temperature and applied to the MEMS device by exposing a surface of the MEMS device to the vapors under vacuum conditions.
However, when the shape of the fixed electrode 1510 and the movable electrode 1520 is complicated as depicted in FIG. 15, it is difficult to coat the electrodes uniformly. In addition, since high voltage is applied to the fixed electrode 1510 and the movable electrode 1520, insulating film coating the electrodes may cause residual polarization; even after the voltage is changed, polarization before the change of voltage remains. In this case, even if the voltage is controlled, actual displacement and planned displacement do not coincide.